The present invention relates generally to magnetic resonance imaging systems, and specifically to systems using steady-state free precession techniques.
Magnetic resonance imaging (MRI) images nuclei having a magnetic moment, usually hydrogen nuclei, by measuring a signal generated by the nuclei precessing in a magnetic field. The angular frequency of precession xcfx890 is directly dependent on the magnetic field B0 within which the nuclei are positioned, according to the Larmor equation:
xcfx890=xcex3xc2x7B0xe2x80x83xe2x80x83(1) 
wherein xcex3 is a constant termed the gyromagnetic ratio.
The magnetic field is set to vary in a known, spatially-dependent manner within the region being imaged, so that the corresponding precession frequency will vary in the same spatially-dependent manner. The spatially-dependent field is generated by imposing a plurality of magnetic fields having known gradients on the homogeneous xe2x80x9cunderlyingxe2x80x9d magnetic field B0. Most preferably, three orthogonal, substantially linear gradients Gx, Gy, and Gz are imposed, so that the magnetic field at any point (x, y, z) is given by the equation:
B(x,y,z)=B0xxc2x7Gx+yxc2x7Gy+zxc2x7Gzxe2x80x83xe2x80x83(2) 
In order to cause nuclei to precess, the nuclei are shifted from their equilibrium thermal state by a pulsed radio-frequency (RF) excitation field, whose magnetic component is in a direction orthogonal to the spatially-dependent magnetic field imposed on the nuclei, herein assumed to be along the z-axis. The frequency is approximately equal to the Larmor frequency, so that the RF pulse acts as a resonant driver of the nuclei. At the conclusion of the RF driving pulse, the nuclei will have been xe2x80x9cflippedxe2x80x9d towards the x-y plane, by an angle dependent on the length and amplitude of the RF pulse. The nuclei then relax towards their thermal equilibrium state, by precessing about the magnetic field, and thus generate a precession signal. The intensity of a specific frequency of precession signal will be a function of the numbers of nuclei precessing at that frequency, and thus the intensity gives a measure of the density of those nuclei at the position defined by the frequency.
Steady-state free precession (SSFP) is a technique for generating MRI signals which is well known in the MRI art, wherein the hydrogen nuclei do not completely return to their thermal equilibrium state. SSFP pulse sequences are described in Magnetic Resonance Imaging by Haacke et al., published by Wiley-Liss. The technique relies on achieving a quasi-steady-state of magnetization in a subject being scanned, usually a human subject, by applying an SSFP pulse sequence at repetition times (TR) significantly shorter than the spin-lattice (T1) and the spinxe2x80x94spin (T2) relaxation times of hydrogen nuclei within the subject. The SSFP pulse sequence comprises a series of RF excitation pulses. The SSFP sequence also comprises a plurality of magnetic gradient pulses which reverse the magnetic field gradients in a predetermined manner, in order to enhance the signal, by methods which are known in the art. Each set of pulses has the same overall repetition time TR. Using SSFP pulse sequences achieves high signal-to-noise ratios within short scan times. However, images produced by some SSFP sequences are very sensitive to motion.
An article titled xe2x80x9cMotion-Insensitive, Steady-State Free Precession Imaging,xe2x80x9d by Zur et al., in Magnetic Resonance in Medicine 16 (1990), which is incorporated herein by reference, describes a method for overcoming problems associated with SSFP sequences caused by motion of the region being scanned. The method comprises generating the magnetic field gradients so that a time integral of each of the gradients during a TR period is substantially zero.
The method further comprises changing a phase of a transverse magnetization of the nuclei in a sequential manner, most preferably by changing a phase of the excitation pulses. For a series of N scan sequences, a phase shift of       2    ⁢          π      ⁡              (                  j          -          1                )              N
radians is added, as explained in appendix B, to the spins in each TR during the jth sequence (j=1, 2, . . . , N). The signals from these scans are linearly combined to obtain a final image. The authors state that to avoid aliasing, it is necessary to use Nxe2x89xa76, and in order to reach steady-state it is necessary to wait T1 seconds between sequences. The authors further state that the SSFP signals are strongly dependent on the angle of precession, xcfx86, where xcfx86 is the total precession angle over one TR period.
In addition to determining the density of hydrogen nuclei at different sections of a region being imaged, the ability to differentiate between molecular species within which the hydrogen is a component is important. Methods for generating MRI scans which differentiate between species, such as water and fat, in an image are known in the art. For example, in an article titled xe2x80x9cLinear Combination Steady-State Free Precession MRIxe2x80x9d by Vasanawala et al., in Magnetic Resonance in Medicine 43 (2000), which is incorporated herein by reference, the authors describe a method for differentiating between water and fat by performing a series of SSFP scans. A first scan sequence is set to be a standard SSFP sequence, and generates raw data termed D0-0. In a second scan sequence a phase of 180xc2x0 is added to even numbered RF excitation pulses, generating raw data termed D0-180. A water image is obtained from D0-0+ixc2x7D0-180; a fat image is obtained from D0-0xe2x88x92ixc2x7D0-180. Unfortunately, the separation of water from fat is affected both by the value of   T1  T2
of the sample and by the RF flip angle. Furthermore, in this method, the value of TR is restricted to:                     TR        =                  1                      2            ⁢                          Δ              WF                                                          (        3        )            
wherein xcex94WF is a difference between water and fat resonant frequencies, and the method is unable to determine water and fat content in a single voxel.
In general, in an imaging volume strong banding artifacts are generated if xcex94"PHgr", the variation in precession angle "PHgr" within the volume, is greater than about xcfx80 radians. Since xcex94"PHgr"=2xcfx80xc2x7≢fxc2x7TR, artifacts do not occur if:                     TR         less than                   1                      2            ⁢            Δ            ⁢                          xe2x80x83                        ⁢            f                                              (        4        )            
wherein xcex94f is the resonance frequency variation in the imaging volume.
While values of TR satisfying inequality (4) are possible at low fields, at higher fields, i.e., approximately 1.0 T and above, the required short TRs cause severe practical problems of implementation. The short TRs necessitate very short gradient switching times and very short image signal acquisition times. Thus, the known advantages of higher-field MRI are difficult to implement with short values of TR, which also has the effect of generating peripheral nerve stimulation and an increase in RF specific absorption rate (SAR).
Disadvantages of short TR include 1) High gradient demand. The maximum available in-plane resolution and slice width is very restricted. 2) Sub-optimal SNR per unit time, because the time allotted for data acquisition in each TR is short. 3) Efficient k-space acquisition strategies such as spiral and multi-shot EPI cannot be used. 4) Fat signal suppression is difficult. 5) SAR is high.
It is therefore desirable to provide apparatus and methods for generating magnetic resonance images without a restriction of repetition time.
In preferred embodiments of the present invention, a magnetic resonance imaging (MRI) system is implemented using radio-frequency (RF) and magnetic gradient pulses in a set of SSFP sequences. Each SSFP sequence comprises a short repetition time (TR) gradient echo with fully balanced gradients in the sequence. A set of MRI generating signals comprises two to five, most preferably two or three, SSFP sequences with RF excitation pulses having high flip angles. The repetition time for each sequence is not limited to short values. By applying specific signal acquisition and analysis techniques, described hereinbelow, and by using flip angles close to 90xc2x0, inaccuracies caused by not utilizing six or more SSFP sequences as a set of generating signals, as are used in the prior art, are significantly minimized for all tissues, and especially for tissues with short spinxe2x80x94spin relaxation times (T2). Image signal variation vs. precession angle is reduced enough so that short repetition times, as required by the prior art, are no longer required. Using flip angles close to 90xc2x0 generates the added advantage, compared to methods used in the prior art, of providing a very high contrast when imaging fluid and soft tissue.
In preferred embodiments of the present invention, a set of N SSFP scans is acquired respectively from N sets of SSFP sequences. Most preferably, N=2. Alternatively, N is a whole number chosen from {3, 4, 5}. An incremental phase is added between scans of each set of sequences, as described in the Background of the Invention. A set of images, most preferably 2 images, is generated via a linear combination of the N acquired data sets. Preferably, the linear combination is formed from the xe2x80x9crawxe2x80x9d data sets and is then reconstructed to form the images. Alternatively, the linear combination is formed after each data set has been reconstruction. The magnitudes of the images are added to provide a final image having a higher signal-to-noise level compared with the separate images.
In some preferred embodiments of the present invention, the RF pulse in the first sequence of each set of sequences is preceded by an RF pre-pulse, and immediately afterwards a de-phasing magnetic gradient pulse is applied to the system being imaged. The combination of RF pre-pulse and de-phasing gradient effectively zeroes the magnetization of the system prior to the subsequent RF pulse. The system approaches a steady state in a substantially smooth manner, enabling measurements made on the system to be utilized from the initial RF pulses.
There is therefore provided, according to a preferred embodiment of the present invention, a method for magnetic resonance imaging (MRI), including:
imposing N sets of steady-state free precession (SSFP) sequences on an object to be imaged, the sequences comprising respective initial radio-frequency (RF) excitation pulses, each initial RF excitation pulse having a predetermined phase shift relative to the other initial RF excitation pulses, wherein N is a value chosen from a set of whole numbers larger than one and less than six;
setting the phase shift of the RF pulse of the sequences so that the phase shift of the RF pulse of an Mth sequence is substantially equal to       2    ⁢          π      ⁡              (                  M          -          1                )              N
radians, wherein M is a value chosen from a set of whole numbers larger than 0 and less than or equal to N;
receiving a respective set of image signals from the object responsive to the N sets of SSFP sequences; and
processing the set of received image signals so as to generate an image of the object.
Preferably, processing the set of received image signals includes:
combining the image signals to form a first linear combination and a second linear combination thereof; and
generating the image by averaging a first magnitude of the first linear combination and a second magnitude of the second linear combination.
Further preferably, processing the set of received image signals includes performing a Fourier transform on each of the image signals, and combining the image signals includes combining the Fourier transforms.
Preferably, at least some of the image signals are dependent on an angle of precession "PHgr", and the image of the object is substantially independent of "PHgr", so that substantially no banding artifacts occur in the image.
Alternatively or additionally, the object includes includes a region having a resonant frequency varying by a factor xcex94f, and a repetition time (TR) of each sequence of the N sets of SSFP sequences includes a time greater than a reciprocal of 2xcex94f.
Preferably, each of the RF excitation pulses generates a flip angle greater than about 70xc2x0.
Preferably, the object includes body fluids and soft tissues, and the image of the object includes respective regions corresponding to the body fluids and the soft tissues having high contrast between the regions.
Further preferably, the first SSFP sequence in each of the N sets of sequences is preceded by a de-phasing magnetic gradient and an RF pre-pulse which generates a flip angle substantially equal to 90xc2x0.
Preferably, the object includes water and fat, and the method includes:
setting a frequency of a frequency synthesizer generating the N sets of SSFP sequences to be substantially equal to an average of a water resonance frequency xcexdW and a fat resonance frequency xcexdF;
setting a repetition time (TR) of each of the sequences to be substantially equal to a first odd integral of a time period xcfx84, wherein       τ    =          1              2        ⁢                  (                                    ν              F                        -                          ν              W                                )                      ;
wherein receiving the respective set of image signals includes:
receiving a first set of image signals at a first readout time substantially equal to a second odd integral of the time period xcfx84 and less than the first odd integral;
receiving a second set of image signals at a second readout time substantially equal to the second odd integral incremented by xcfx84;
wherein processing the set of received image signals includes:
processing the first and the second set of image signals to form respective processed first and second signals; and
generating a water image and a fat image responsive to the first processed signals and the second processed signals.
There is further provided, according to a preferred embodiment of the present invention, apparatus for magnetic resonance imaging (MRI), including:
a magnetic field generator which is adapted to impose N sets of steady-state free precession (SSFP) sequences on an object to be imaged, the sequences comprising respective initial radio-frequency (RF) excitation pulses, each initial RF excitation pulse having a predetermined phase shift relative to the other initial RF excitation pulses, wherein N is a value chosen from a set of whole numbers larger than one and less than six, and wherein the phase shift of the RF pulse of an Mth sequence is substantially equal to       2    ⁢          π      ⁡              (                  M          -          1                )              N
radians, wherein M is chosen from a set of whole numbers larger than 0 and less than or equal to N; and
a signal processor which is adapted to receive a respective set of image signals from the object responsive to the N sets of SSFP sequences, and to process the set of received image signals so as to generate an image of the object.
Preferably, the signal processor is adapted to:
combine the image signals to form a first linear combination and a second linear combination thereof; and
generate the image by averaging a first magnitude of the first linear combination and a second magnitude of the second linear combination.
Further preferably, the signal processor is adapted to perform a Fourier transform on each of the image signals and to combine the Fourier transforms.
Preferably, at least some of the image signals are dependent on an angle of precession "PHgr", and the image of the object is substantially independent of "PHgr", so that substantially no banding artifacts occur in the image.
Alternatively or additionally, the object includes a region varying in resonant frequency by a factor xcex94f, and a repetition time (TR) of each sequence of the N sets of SSFP sequences includes a time greater than a reciprocal of 2xcex94f.
Preferably, each of the RF excitation pulses generates a flip angle greater than about 70xc2x0.
Preferably, the object includes body fluids and soft tissues, and the image of the object includes respective regions corresponding to the body fluids and the soft tissues having high contrast between the regions.
Preferably, the first SSFP sequence in each of the N sets of sequences is preceded by a de-phasing magnetic gradient and an RF pre-pulse which generates a flip angle substantially equal to 90xc2x0.
Preferably, the object includes water and fat, and the magnetic field generator is adapted to:
set a frequency of a frequency synthesizer generating the N sets of SSFP sequences to be substantially equal to an average of a water resonance frequency xcexdW and a fat resonance frequency xcexdF;
set a repetition time (TR) of each of the sequences to be substantially equal to a first odd integral of a time period xcfx84, wherein       τ    =          1              2        ⁢                  (                                    ν              F                        -                          ν              W                                )                      ;
xe2x80x83and the signal processor is adapted to:
receive a first set of image signals at a first readout time substantially equal to a second odd integral of the time period xcfx84and less than the first odd integral;
receive a second set of image signals at a second readout time substantially equal to the second odd integral incremented by xcfx84;
process the first and the second set of image signals to form respective processed first and second signals; and
generate a water image and a fat image responsive to the first processed signals and the second processed signals.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: